1. Field of the Invention
The present invention relates to a multichannel receiver for receiving a plurality of different channel signals at the same time.
2. Description of the Related Art
Typically, there has been wide use of a FFT (Fast Fourier Transform) for simplifying a process for receiving a plurality of channel signals having different carrier frequencies at the same time. Provided that a plurality of channel signals having different carrier frequencies are simultaneously received using the FFT, a Nyquist bandwidth is divided by N/2 when a reception signal is a real digital signal, or divided by N when a reception signal is a complex digital signal, such that N signals having different carrier frequencies are divided for every carrier (i.e., is set to different channels for every carrier) and at the same time each signal can be converted into a baseband signal.
The term “channel” means a frequency domain on a frequency axis for indicating either a reception signal in the communication field and a transmission/reception signal in the broadcasting field, or a carrier frequency of these signal waves.
For example, FIG. 9 is a block diagram illustrating a conventional receiver. Referring to FIG. 9, a quadrature detector 2 includes a first multiplier 22 for multiplying a signal received from an antenna 1 by a real number axis signal “cos” of a local signal generated from a quadrature carrier oscillator 21, and a second multiplier 23 for multiplying the signal received from the antenna 1 by an imaginary number axis signal “−sin” having a phase that is 90 degrees ahead of that of the real number axis signal “cos”, and therefore, converts a plurality of channel signals into complex IF (Intermediate Frequency) signals. Analog-to-digital converters (ADCs) 3a and 3b performs A/D conversion on the complex IF signals of the quadrature detector 2, such that the complex IF signals are converted into complex IF digital signals. FFT 51 receives the complex IF digital signals having different carrier frequencies from the ADCs 3a and 3b, simultaneously converts them into baseband signals, and then outputs the baseband signals to a parallel/serial converter 52.
However, when of receiving a plurality of channel signals having different carrier frequencies using the aforementioned FFT, it is necessary for symbol timings of modulated waves of individual channel signals to be synchronized with each other. Moreover, in order to separate each channel signal from the plurality of channel signals received at the same time, a frequency denoted by an inverse number of the symbol timing (i.e., a symbol synchronization) of the modulated wave should be set to a carrier interval frequency of each channel. In order to meet the above requirements, a prescribed relationship between an OFDM symbol and each modulated signal should be engaged in advance. However, provided that the modulated signals are different in carrier frequency, it is difficult for even an OFDM scheme to meet the above requirements, and moreover there is no way to solve this problem in actual practice.
Therefore, it is desirable that a plurality of receivers are arranged in parallel with one another to receive a plurality of different channel signals at the same time. The FFT scheme is especially suitable to the case where more than several tens of carrier signals are received at the same time, but it is hard to say that the FFT scheme is also suitable to the case where there is a small number of carriers. Moreover, there is no way for a conventional reception end to stably receive a small number of carrier signals at the same time.